2-phenylethanol (2-PEA) is the main component of the rose oils obtained from rose blossoms. 2-phenylethanol is a colorless liquid possessing a faint but lasting odor of rose petals, making it as a valuable chemical of commerce. 2-PEA is extensively used in perfumery, soaps and detergents, deodorant formulations and as food additive [B. D. Mookherjee, R. A. Wilson, Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., vol. 17, John Wiley & Sons, New York, 1994]. 2-PEA has also bacteriostatic and antifungicidal properties and is frequently used in the formulations of cosmetics. Besides its direct application 2-PEA is also used as an intermediate in the synthesis of industrially important perfumery chemical compounds like synthetic Kewra (2-phenylethyl methyl ether). Due to the commercial importance of 2-phenyl ethanol various methods have been reported for its production.
Friedel-Crafts alkylation of benzene using ethylene oxide and stoichiometric quantities of aluminum chloride as catalyst has been used to produce 2-PEA on commercial scale [K. Bauer, D. Garbe, H. Surburg in Common Fragrance and Flavour Materials, New York, (1990), G. A. Olah, V. Prakash Reddy, G. K. Surya Prakash in Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., vol. 11, John Wiley & Sons, New York, (1994)]. Major disadvantages of this process are the fact that (i) Friedel-Crafts catalyst (AlCl3) is corrosive hence requires the expansive corrosion resistance equipments, (ii) produces environmentally hazardous effluent in large quantities which causes effluent disposal problems and (iii) complete removal of the catalyst from the product is difficult.
2-phenylethanol is also prepared by the Grignard synthesis, starting from chlorobenzene, which is converted to phenyl-magnesium chloride, which then reacts with ethylene oxide to give phenylethoxy magnesium chloride. Phenylethoxy magnesium chloride thus obtained is decomposed with sulphuric acid to give 2-PEA. The main drawbacks of this process are (i) generation of potentially dangerous and air and moister sensitive phenyl-magnesium chloride (ii) requirement of sulphuric acid for the decomposition of Grignard complex and subsequent extraction of the desired product, (iii) produces large amounts of hazardous effluent and poor quality of the 2-phenylethanol which has low acceptability in perfumery industry [E. T. Theimer in Fragrance Chemistry, Academic Press, New York, (1982) p. 271].
The reductive cleavage of epoxides to alcohols is one of the most useful reactions in organic synthesis [H. Fujitsu, S. Shirahama, E. Matsumura, K. Takeshita, I. Mochida, J. Org. Chem. 46 (1981) 2287; S. Krishnamurthy, R. M. Schubert H. C. Brown, J. Am. Chem. Soc. 95 (1973) 8486]. In this process the required alcohol can be obtained by the ring opening of the corresponding epoxide using various hydrogenating agents [M. Bartok, K. L. Lang, in: A. Weissberger, E. C. Taylor (Eds.), The Chemistry of Heterocyclic Compounds-Small Ring Heterocycles, Wiley, New York, 1985] such as hydrides of alkaline metals like LiAlH4, LiAlH4/AlCl3, NaBH4, B2H6, LiInH4, LiPhInH3, LiPhInH2, Borane/morpholine and zeolites/silica supported Zn(BH4)2 [M. Bartok, K. L. Lang, in: A. Weissberger, E. C. Taylor (Eds.), The Chemistry of Heterocyclic Compounds—Small Ring Heterocycles, Wiley, New York, 1985; G. Smith, Janice, Synthesis 8 (1984) 629; C. Bonini, R. D. Fabio, G. Sotgiu, S. Cavagnero, Tetrahedron 45 (1989) 2895-2904; M. Yamada, K. Tanaka, S. Araki, Y. Butsugan, Tetrahetdron Letters 36 (1995) 3169; W. B. Smith, J. Org. Chem. 49 (1984) 3219; R. Sreekumar, R. Padmakurmar, R. P. Rugmini, Tetrahedron Lett. 39 (1998) 5151; B. C. Ranu, A. R. Das, J. Chem. Soc. Chem. Commun. (1990) 1334]. The drawbacks of above stated methods are; (i) the product selectivity for the desired primary alcohol is poor, (ii) generates large of amount of salts which hampers effective separation of the product from the reaction mixture, (iii) All the above mentioned hydrogenating agents are expensive and (iv) potentially hazardous and need extra care for storage.
Several hydrogenation catalysts such as, Raney nickel, supported palladium and platinum catalysts have been reported to be good catalysts for the hydrogenation of styrene oxide [Wood, U.S. Pat. No. 2,524,096; Hopff et al., U.S. Pat. No. 2,822,403; U.S. Rubber Co., British Pat. Spec. No. 678,589; Wood U.S. Pat. No. 3,579,593; Gibson et al., U.S. Pat. No. 4,064,186; Hoeldrich et al. U.S. Pat. No. 4,943,667; and Frisch, Canadian Pat. No. 512236. F Fujitsu, S. Shirahama, E. Matsumura, K. Takeshita, I. Mochida, J. Org. Chem. 46 (1981) 2287; C. V. Rode, M. M. Telkar, R. Jaganathan, R. V. Chaudhari, J. Mol. Catal A: Chem. 200 (2003) 279]. This reaction is also considered as most atom economical and environmentally benign method to produce 2-PEA. However, hydrogenation of styrene oxide is usually accompanied with formation of several side products such as phenyl acetaldehyde, 1-phenyl ethanol, styrene and ethyl benzene. These by-products when present in even a small amount may destroy the aroma of the 2-PEA, thus making it unsuitable for perfumery formulations.
Hopff et al. in U.S. Pat. No. 2,822,403, used a combination of Raney nickel and other hydrogenating catalyst like Cobalt, Platinum and Palladium for the catalytic hydrogenation of styrene oxide was then carried out in the presence of water and emulsifying agent to achieve high yield. However, this process has several disadvantages (i) need to remove large amount of water, (2) additional steps like solvent extraction, salting out process is necessary for product separation and (3) formation of ethyl benzene in large quantity.
A British Pat. No. 760768 describes the hydrogenation of styrene oxide with Raney nickel alone instead of combination of two catalysts using reaction conditions as given in preceding paragraph. This process suffers similar disadvantages as above.
Wood et al., in U.S. Pat. No. 3,579,593 described hydrogenation catalysts having varying content of Raney nickel and palladium in order to achieve high 2-PEA selectivity. However, to get higher 2-PEA selectivity (˜96%) two stage temperature (stage 1. 30-40° C.; stage 2. 90-110° C.) and pressure (stage 1. ˜50 psi; stage 2. ˜200 psi) operation was required.
Recently, improved process for preparation of 2-PEA was also described in U.S. Pat. No. 6,166,269. This process produced 2-PEA with high selectivity via one step the catalytic hydrogenation of styrene oxide using platinum group metal catalysts supported on carbon/alumina in the presence of various organic/inorganic bases as promoter. However, the main disadvantages of this process are (i) requirement of organic and inorganic base promoters to increase 2-phenyl ethanol selectivity, (ii) removal of organic and inorganic base promoters after completion of reaction and (iii) requires large amounts of water to remove base promoters.
Several publications were dedicated on the use of Pd on carbon or Raney nickel as hydrogenation catalyst in presence or absence of a base promoter for the preparation of 2-PEA from styrene oxide. However, both these catalysts are highly pyrophoric, air sensitive and particularly nickel is toxic. [S. Mitsui, S. Imaizumi, M. Hisashige, Y. Sugi, Tetrahedron Lett. 29 (1973) 4093; V. G. Yadav, S. B. Chandalia, Org. Process Res. Dev. 2 (1998) 294; C. V. Rode, M. M. Telkar, R. Jaganathan, R. V. Chaudhari, J. Mol. Cat. A: Chemical 200 (2003) 279].
Olga Bergada et al. described the use of Ni supported on MgO [Appl. Catal. 272 (2004) 125] and hydrotalcite (Ni/Mg/Al) [Appl. Catal. 331 (2007) 19] for the hydrogenation of styrene oxide to give 2-PEA in high yield and selectivity. But a major drawback of these catalysts is that a strict air free condition is required to handle/store the catalyst. Further, to get the active catalyst the catalyst precursors were subjected to heating at very high temperature (350° C.) for 4 h with H2 which is potentially hazardous.
Kirm et al. studied the gas phase hydrogenation of styrene oxide where vapors of hydrogen and styrene oxide in 20:1 molar ratio was passed over catalyst (Pd supported on activated carbon, MgO, γ-Al2O3) in a fixed bed reactor at temperature above 75° C. at a space velocity of 10,000-35,000 h−1. They observed that high selectivity of 2-PEA was obtained in the case of basic support [J. Mol. Cat. A: Chemical 239 (2005) 215]. However, the drawbacks of above catalyst systems are; (i) the requirement of either special device for the reduction of Pd precursors using H2 gas at 350° C. (ii) the catalyst thus prepared needed to store under air-free conditions (iii) need to use large excess of hydrogen.
The above discussed processes suffer from various obstacles such as use of hazardous/toxic reagents/catalysts, produces large amount of effluent, separation of the catalyst used, low selectivity of 2-PEA, requirement of basic promoters to enhance selectivity of 2-PEA, requirement of special device or chemical reagent for reduction of the metal precursors used, catalyst sensitivity, stability and catalyst recycling.
In view of this, we have developed a catalyst where Pd (II) was loaded on basic inorganic support. The supported Pd (II) catalysts thus prepared generate Pd (0) in situ to catalyze hydrogenation of epoxide substrate. Therefore, an additional step of metal reduction was avoided. Additionally Pd (II) catalysts are non-pyrophoric (do not catch fire in air) and sufficiently stable under ambient condition (no change in catalyst performance was observed over a period of six months when stored in a tightly closed bottle at room temperature whose temperature ranged varied over 20-35° C. under atmospheric condition that is to say, no artificially created inert atmosphere). Which makes the handling easier and do not require special care for their storage. Further, Pd (II) supported on basic inorganic supports are heavier than conventionally used Pd (0) on carbon, therefore produce no or little dust. Furthermore, the selectivity for the desired product is very high with as developed catalysts, which is consistent over many catalyst-reuse experiments. Due to these qualities, the Pd/basic inorganic support is suitable for large scale hydrogenation of styrene oxide as demonstrated in some of the examples given in the present patent and have significantly better alternative to the currently used catalyst in terms of economy, safety, and eco-friendly nature.